Internal combustion engines on the basis of Otto engines are generally operated with fuel from hydrocarbons from fossil fuels on the basis of refined crude oil. Ethanol produced from renewable resources (plants) or another alcohol is increasingly being added to this fuel in different mixing ratios. In the USA and Europe a blend of 70-85% ethanol and 15-30% gasoline is often used under the brand name E85. The internal combustion engines are designed in such a way that they can be operated with pure gasoline as well as with blends up to E85. This is designated as a “flex fuel operation”. The operating parameters in the flex fuel operation have to be adapted to the respectively prevailing fuel blend in order for a fuel efficient operation with low pollutant emissions to occur, while a high degree of engine performance and an ease in starting are simultaneously maintained. An exemplary stoichiometric fuel/air ratio exists at 14.7 parts of air to each part of gasoline; however, when using pure ethanol, an air amount of 9 parts has to be set.
When starting Otto engines (spark-ignited internal combustion engines), a high-pressure injection of the fuel is only then enabled when a predefined threshold for the pressure in the injection system is achieved. The reason for this is that a good quality of atomization can only then be achieved at or above said threshold. The high-pressure injection in an Otto engine with direct gasoline injection is, for example, only then enabled if a temperature dependent threshold of the fuel pressure in a fuel distribution line is achieved, as would ideally be the case in a common rail pressure reservoir. A high-pressure start ideally means that a majority of the fuel quantity provided for a combustion stroke is injected during an intake stroke of the internal combustion engine under high pressure into a combustion chamber and causes a homogeneous cylinder charging. In a subsequent compression phase, an additional injection occurs, which causes a stratified charge. Provision can be made for a third injection phase shortly after the ignition point. The fuel quantity provided can also alternatively be deposited in only one injection during the compression phase.
When the pressure is lower than the threshold pressure, the injection in the low pressure operating mode is operated with altered injection parameters. An injection during a low pressure starting mode typically occurs in up to three partial injections, which all occur prior to the ignition point. In so doing, a good distribution of the fuel in the combustion chamber is achieved on the one hand, and furthermore a decrease in the drop in pressure in the common rail is also achieved. The injection cycles can be selected in such a way that an ingress of the gas mixture from the combustion chamber into the injector is prevented despite the low fuel pressure. The threshold value of the fuel pressure can thereby be selected as a function of the temperature of a coolant or of the combustion chamber of the internal combustion engine. When temperatures are low, fuel increasingly condenses on the cylinder walls, is emitted in an uncombusted state into the atmosphere and thereby increases the toxic emissions. Said emissions are reduced during a low-pressure start by a decrease in the initial injected fuel quantity, which decreases the amount of condensation of fuel on the cylinder wall. Provision can be made for the selection of the injection parameters to exclusively cause a stratified charge in the combustion chamber.
Emissions during starting can be reduced and a starting procedure with a reduced likelihood of misfires occurs as a result of the approach described above. A low-pressure start likewise generally occurs below a minimum and above a maximum temperature of, for example, the coolant. The low-pressure start ideally occurs below −15° C. and above 90° C. A high-pressure start occurs in the temperature range between the aforementioned values if a sufficient operating pressure of ideally 10 bar is achieved. If such an Otto engine is operated with a gasoline-ethanol mixture, the composition of said mixture must be taken into consideration during starting in order to achieve a flawless starting behavior. This is not taken into consideration in the technical field.